Often it is desirable to provide a circuit breaker switch in series with a capacitive load so as to selectively interrupt the supplying of DC power to the load. If substantial DC voltages and currents are contemplated, the circuit breaker may have to be capable of substantial power dissipation, particularly during the initial charging up of the capacitance of the load. In prior circuits, an electromechanical relay has been used when power dissipation was a concern. However, such relays have inherent long term reliability problems, are relatively expensive and may be relatively costly and bulky in size if they are required to have a substantial power dissipation/current conduction rating.
A single semiconductor switching device could be utilized in place of the above noted mechanical relay. However, the cost of such a single semiconductor component, which would be able withstand the power dissipation encountered during the initial charging time and conduct sufficient maximum current, would make the use of such a single semiconductor switch undesirable. A semiconductor switch having a lower power rating could be utilized if the semiconductor switch were only gradually or slowly turned on or if the initial charging current through the semiconductor switch was limited by feedback so that only a certain maximum power dissipation would occur in the semiconductor device during the initial charging time. However, these solutions would inherently slow down the charging up the of the capacitive load because the effective switch current multiplied by the voltage across the semiconductor switch would be limited to a maximum power dissipation rating which preferably is selected to be just enough to allow the switch to supply anticipated steady state load current. Also, any feedback current limit solution would be difficult to implement. The implementing difficulties occur because very small currents during the initial charging time could result in a very large power dissipation for the semiconductor switch due to the large voltage differential across the switch, whereas during the later stages of the charging time there is less voltage across the semiconductor switch and therefore a larger current could exist for the same power dissipation rating. Thus any type of feedback system which would limit the power dissipation of the series semiconductor switch to some preset level would have to operate over a very wide range of charging currents, and accurately implementing such switch control would be difficult.
Some prior art circuits have provided a constant resistive charging path in parallel with a semiconductor device in a power supply that provides power to a capacitive load. However, this semiconductor device, and its associated circuitry, have been inserted just to provide a small series voltage drop under some conditions while the resistor limits the DC power supply current under other conditions. Thus these circuits do not implement a circuit breaker function, responsive to fault conditions, which is connected in series between DC power and a capacitive load. This is because the resistor always provides a current bypass to the semiconductor switch. Therefore, this technique does not suggest how to prevent overpower dissipation from occurring in a series circuit breaker type switch while still achieving a relatively rapid charging up of a capacitive type load and/or implementing this without any requirement for costly high power components or extremely complex and close tolerance power dissipation feedback control circuitry.